1. Field of the Invention
This invention relates to a real image mode finder suitable for use in a lens shutter camera or an electronic still camera in which a finder optical system is constructed to be independent of a photographing optical system.
2. Description of Related Art
In general, finders in which a finder optical system is constructed to be independent of a photographing optical system, used in lens shutter cameras, can be roughly divided into two classes: virtual image mode finders and real image mode finders. In the case where an attempt is made to improve a variable magnification ratio of the virtual image mode finder, the diameter of a front lens must be enlarged, which constitutes an obstacle to compactness of the finder. In this way, the real image mode finder is used for preference in a compact camera.
Recently, in keeping with compactness of the lens shutter camera and an improvement on the variable magnification ratio of its finder, further compactness of the finder mounted in the camera has been required. In particular, interest in a so-called depth-reduced camera, which is designed to reduce the depth of the camera (in a direction indicated by an arrow Q in FIG. 1), is heightened. In order to construct such a camera, it is particularly necessary to reduce the overall length of the finder, not to speak of a collapsible length of the photographing optical system. Here, the overall length refers to an actual length from the foremost object-side surface of an objective optical system including an image erecting optical system to the rearmost pupil-side surface of an eyepiece optical system. In addition, for compactness of the finder, a reduction in the thickness of the image erecting optical system, as well as in the overall length of a lens system constituting the finder, is required.
Furthermore, in recent compact cameras, a camera usually referred to as a zoom type in which lenses need not be exchanged and photography can be performed with a single lens system in regard to various focal lengths is being chiefly used, and a higher variable magnification ratio is required. A variable magnification camera with a variable magnification ratio as high as 2.5 or more is also often required.
In order to meet such demands, it is necessary to construct a depth-reduced camera which is small in size and has a high variable magnification ratio.
An image erecting optical system for constituting such a camera includes, for example, a conventional Porro prism, a combination of a roof prism and a pentaprism such as that disclosed in Japanese Patent Preliminary Publication No.. Hei 9-211544, or an optical system disclosed in each of Japanese Patent Preliminary Publication Nos. Hei 3-81749, Hei 3-217829, and Hei 8-129203.
On the other hand, a finder for constituting the camera which is small in size and has a high variable magnification ratio is designed so that an objective optical system includes lenses with negative, positive, and positive powers in this order from the object side, as disclosed in each of Japanese Patent Preliminary Publication Nos. Hei 5-93859 and Hei 8-240769, or an objective optical system includes lenses with negative, positive, and negative powers in this order from the object side, as disclosed in Japanese Patent Preliminary Publication No. Hei 8-76192.
However, if the Porro prism is used as the image erecting optical system of the finder, a finder optical system becomes bulky in a vertical direction when a ray of light is deflected vertically, which causes obstruction to compactness of the camera.
In contrast with this, when the image erecting optical system is constructed with a combination of the roof prism and the pentaprism as disclosed in Hei 9-211544, a vertical space required for the finder optical system is approximately halved, compared with the case where the Porro prism is used, even when the ray of light is deflected vertically as shown in FIG. 2. In this case, however, an angle of deflection .theta. of the ray of light in the roof prism is about 90.degree.. If the angle of deflection .theta. is made smaller than 90.degree., a thickness A of the image erecting optical system will be increased. In general, this indicates that as the angle of deflection .theta. of the ray increases, a space can be saved with respect to a direction along the optical axis of incidence on the finder.
In FIG. 3A, an angle of incidence .gamma. of an axial ray Li on a reflecting surface R is larger than that of FIG. 3B. From these figures, it is found that when light beams with identical diameters are bent, a space C.sub.0 required increases with increasing angle of incidence of the ray on the reflecting surface. Thus, when an image erecting optical system in which the space C.sub.0 is relatively large is used to reduce the thickness of the finder (that is, diminish a distance from the entrance surface of the objective optical system to the exit surface of the eyepiece optical system), it is inevitable that a space (indicated by reference symbol B in FIG. 2) in which the zoom lens units of the objective optical system are movable becomes narrow, which is unfavorable. Specifically, if an attempt is made to forcedly attain a high variable magnification ratio in a narrow zoom space in order to reduce the depth of the camera, the refracting power of each of the lens units of the objective optical system in the finder must be strengthened. This causes considerable degradation to the performance of the finder even with a slight manufacturing error.
Similarly, in an embodiment disclosed in Hei 3-81749, as shown in FIG. 4, the angle of deflection of a ray of light at a first reflecting member 101 is small and thus compactness of the finder is accomplished in terms of its lateral direction, but not in terms of the depth of the camera. Further, In an embodiment disclosed in Hei 3-217829, as shown in FIG. 5, the angle of deflection of a ray at a first reflecting section 102 is large and this is favorable for a reduction in thickness of the finder. Since, however, this finder is designed so that the angle of deflection of the ray at a second reflecting section 103 is also relatively large and an incident ray of the finder is nearly parallel to an emergent ray thereof, the angle of deflection of the ray at a third reflection section 104 must be made small, and the thickness of the finder cannot be completely reduced.
In the disclosure of Hei 8-129203, a light beam is introduced into a roof reflecting section through two reflecting members. In this embodiment, the angles of deflection of the ray at the two planar reflecting members are small and this is suitable for reducing the depth of the camera. With such a configuration, however, the optical path length of the eyepiece optical system must be increased in order to ensure the optical path of the roof reflecting section. For this purpose, if an attempt is made to simply construct the eyepiece optical system, for example, with a single lens, it becomes very difficult to diminish the focal length of the eyepiece optical system. This directly causes a reduction in finder magnification, which is a value dividing the focal length of the objective optical system by that of the eyepiece optical system, and results in a cumbersome finder. This finder also has the drawback that since the roof reflecting section is located closer to the eyepiece side than an intermediate image position, the roof ridgeline of the roof reflecting section enters the visual field even when the eye is slightly separated from the camera.
For the objective optical system, a smaller zoom space is suitable for compactness of the camera because of a restriction on the layout of the camera.
In each of the embodiments disclosed in Hei 5-93859 and Hei 8-240769, the objective optical system is constructed with lens units having negative, positive, and positive refracting powers in this order from the object side. Although this power distribution makes the first lens unit easy to fix when the magnification of the finder is changed, the back focal distance of the objective optical system becomes diminished. Thus, when the roof reflecting section is particularly interposed between the objective optical system and the intermediate image position thereof, the roof reflecting section generally needs an optical path length longer than the case where a ray of light is twice-reflected by planar reflecting surfaces. Consequently, its arrangement becomes difficult or a severe restriction is imposed on the angle of reflection of the ray.
A finder set forth in each of the first to third embodiments of Hei 8-76192 is constructed with lens units having negative, positive, and negative refracting powers in this order from the object side, and has the power distribution which allows the third lens unit to be fixed when the magnification is changed. On the other hand, this finder, which is designed to need a large zoom space, is not suitable for compactness or a reduction of thickness. A finder of the fourth embodiment is constructed to attain a high variable magnification ratio with a narrow zoom space, but distortion produced in the vicinity of the wide-angle position of the finder is as much as 15% and thus it cannot be positively said that complete optical performance is maintained. Further, a single lens is used as a condenser lens in the vicinity of the intermediate image position. In this way, when the single lens is properly used, good optical performance is easier to obtain than the case where the entrance surface of a prism is provided with a curvature to possess the function of the condenser lens. This arrangement, however, brings about a costly finder which has a large number of parts. Since the number of surfaces of optical members are also increased, the transmittance of light is reduced and a finder which is dark in visual field and hard to see is obtained. Additionally, in this embodiment, there is the need to construct an image erecting optical system using mirrors between the objective optical system and the intermediate image position. In general, however, a mirror reflecting surface has a reflectance lower than the totally reflecting surface of a prism, and thus there is a high possibility that the finder becomes darker.